What is the suffix for word great?

The word "great" doesn't have a suffix. It's a stand-alone word.

However, there are suffixes that can be added to "great" to form new words, such as:

* -ness: Greatness

* -ly: Greatly

* -ful: Grateful

* -er: Greater

* -est: Greatest

These suffixes change the meaning and grammatical function of the word.